The present invention generally relates to a light beam scanning apparatus, and in particular to a light beam scanning apparatus in which a recording medium is scanned by using a plurality of light beams emitted simultaneously. The present invention relates, more particularly, to a multi-beam detecting apparatus built in the light beam scanning apparatus for separately detecting a plurality of light beams by using a single photodetector. Such a multi-beam detecting apparatus is applicable to a light beam scanning apparatus such as printers, copiers and facsimile machines in order to control modulation start timing for the light beam.
Recently, optical systems such as printers, copiers and facsimile machines which utilize a laser beam has come into wide use. A laser beam is used for writing information on a recording medium such as a photosensitive drum. The use of a laser beam makes it possible to increase the scanning speed and enhance the resolution of printed images. In such systems, it is required to deflect a laser beam in order to scan the recording medium. In conventional printers and copiers, polygon scanners or polygon mirrors are widely used as light deflectors, and contributes to increasing the scanning speed. However, there is a limited rotational speed of such optical deflectors in view of mechanical structures thereof. Therefore, currently, there is considerable activity in the development of multi-beam scanning systems in which a plurality of light beams are simultaneously deflected and is then irradiated on a recording medium. With such multi-beam scanning systems, it becomes possible to increase the scanning speed without increasing the rotational speed of optical deflectors.
FIG. 1 shows an example of a conventional optical beam scanning apparatus. The illustrated scanning apparatus comprises the following structural parts. A light source array 1 is constituted by a laser diode array capable of emitting a plurality of laser beams at the same time (three beams in the illustrated example). A collimator lens 2 forms the laser beams emitted from the light source unit 1 to parallel laser beams. An optical deflector 3 is constituted by a polygon mirror. The polygon mirror 3 rotates in the direction indicated by an arrow A3, and deflects the parallel laser beams from the collimator lens 2 in a main scanning direction indicated by an arrow A1. Thereby, the deflected parallel laser beams, or scanning beams repetitively scan a recording medium 7 of a photosensitive drum in the main scanning direction of the arrow A1. An image formation lens 4 focuses each of the scanning beams so as to have predetermined diameters and be arranged, with a predetermined pitch, on a circumferential surface of the photosensitive drum 7. A reflection mirror 5 is provided for guiding the scanning beams from the polygon mirror 3 toward a photodetector 6. The photodetector 6 receives the scanning beams reflected on the reflection mirror 5.
In the optical beam scanning apparatus thus constructed, the photosensitive drum 7 is raster-scanned. That is, the photosensitive drum 7 is repetitively scanned in the main scanning direction of the arrow A1 by the deflected laser beams, while driven in a sub scanning direction indicated by the direction of an arrow A2. In a case where the photosensitive drum 7 is substituted with a plate-shaped recording medium, it is driven linearly.
With the above-mentioned structure, it becomes possible to write information on the corresponding plurality of lines at one time. As a result, it becomes possible to increase the speed of the sub scan multiple times corresponding to the number of the plurality of scanning beams. Further, it is unnecessary to alter a pixel clock used for modulating laser beams.
The photodetector 6 is served as a synchronization detecting means, which detects an event that the laser beams come to a predetermined position, and simultaneously starts a modulation control for the laser beams. By using the photodetector 6, it becomes possible to establish a synchronization for the laser beams, even if there are fluctuations in deflection arising from states of planes of the polygon mirror 3. In other words, modulation (writing) starting times (positions) for the respective laser beams can be compensated.
In order to establish the above-mentioned synchronization, it is required to separately detect the scanning beams with high accuracy. In multi-beam systems, it is required to set a distance between loci of scanning beams equal to a pitch for a desired write density. It is further required that the scanning beams are detected in the vicinity of the peripheral surface of the photosensitive drum 7. In order to meet the above-mentioned requirements, it is known to arrange independent photodetectors as many as the number of laser beams. However, this is not suitable for practical use, because it is necessary for the photodetectors to be actually arranged with a designed write pitch.
In order to overcome the above-mentioned problem, it is also known to emit a plurality of laser beams so as to be arranged into a line in the sub scanning direction. However, there is a limitation on the beam emission pitch of the laser diode unit 1. Currently, it is very difficult to emit a plurality of laser beams with a beam emission pitch corresponding to a high write density equal to or greater than 300 dots per inch. For this reason, actually, a laser diode array is constructed by a plurality of laser diodes arranged with a pitch greater than a desired beam emission pitch, and is positioned so as to be inclined at a predetermined angle with respect to the sub scanning direction. With the above laser diode array, it becomes possible to obtain on the circumferential surface of the photosensitive drum 7, a pitch between adjacent beams corresponding to the desired write density.
It is noted that in the above-mentioned case, the laser beams do not arrive at the photodetector 6 at the same time, or in other words, there is a differences in the incoming time between the laser beams. For this reason, it is necessary to control a modulation (writing) starting time (position) for each of the laser beams. For this purpose, it is particularly desired to separately detect laser beams with high accuracy.
For example, the Japanese Laid-Open Patent Application No. 57-67375 proposes the use of a light interrupting plate, or slit plate, which is interposed between the light source unit 1 and the photodetector 6. The proposed slit plate has a single slit extending in the sub scanning direction. The slit plate functions to separately detect the incoming of the plurality of scanning laser beams.
FIG. 2 is a view for explaining the principle of the conventional detection scheme by use of the slit plate. As shown, spots of four laser beams B1-B4 are arranged into a line so as to be inclined at an angle .theta. with respect to a line Y--Y perpendicular to the main scanning direction A1. A slit plate having a slit S of a width Ds is placed in the front of the photodetector 6. The synchronization necessary for modulating the light beams is established by detecting light beams which pass through the slit S.
However, the prior art disclosed in the publication has disadvantages described below. It is impossible to make the width Ds of the slit S larger than a beam interval Da, which is determined depending on the pitch of the arrangement of the laser diodes and diameters of spots of the laser beams B1 to B4. Therefore, there is a limited quantity of light which can be received by the photodetector 6. Particularly, as the laser beams scan the photosensitive drum 7 faster, the photodetector 6 receives a decreased amount of light. This deteriorates the detection of light.
In order to eliminate the above problem, it is conceivable to increase a distance Da between opposite edges of adjacent spots of laser beams to thereby increase a distance Db between the centers of the adjacent beams. However, the above causes some problems in view of design of an optical system. For example, it is necessary to increase areas of deflection surfaces of the polygon mirror 6.
It is preferable that the oblique angle .theta. of the laser diode array built in the laser diode unit 1 is made as small as possible in cases when giving the priority to the design of optical systems in order to obtain a simple structure thereof while taking into consideration costs and performance. Therefore, an improved multi-beam detecting apparatus is desired capable of separately detecting a plurality of scanning beams which are arranged with a very small value of the distance Da.